Partial purification and characterization of DNA polymerase beta-like enzyme from Plasmodium falciparum

Uracil moieties in Plasmodium falciparum genomic DNA

Plasmodium falciparum parasites undergo multiple genome duplication events during their development. Within the intraerythrocytic stages, parasites encounter an oxidative environment and DNA synthesis necessarily proceeds under these circumstances. In addition to these conditions, the extreme AT bias of the P. falciparum genome poses further constraints for DNA synthesis. Taken together, these circumstances may allow appearance of damaged bases in the PlasmodiumDNA. Here, we focus on uracil that may arise in DNA either via oxidative deamination or thymine‐replacing incorporation. We determine the level of uracil at the ring, trophozoite, and schizont intraerythrocytic stages and evaluate the base‐excision repair potential of P. falciparum to deal with uracil‐DNA repair. We find approximately 7–10 uracil per million bases in the different parasite stages. This level is considerably higher than found in other wild‐type organisms from bacteria to mammalian species. Based on a systematic assessment of P. falciparum genome and transcriptome databases, we conclude that uracil‐DNA repair relies on one single uracil‐DNA glycosylase and proceeds through the long‐patch base‐excision repair route. Although potentially efficient, the repair route still leaves considerable level of uracils in parasite DNA, which may contribute to mutation rates in P. falciparum.

DNA damage regulation and its role in drug-related phenotypes in the malaria parasites

DNA of malaria parasites, Plasmodium falciparum, is subjected to extraordinary high levels of genotoxic insults during its complex life cycle within both the mosquito and human host. Accordingly, most of the components of DNA repair machinery are conserved in the parasite genome. Here, we investigated the genome-wide responses of P. falciparum to DNA damaging agents and provided transcriptional evidence of the existence of the double strand break and excision repair system. We also showed that acetylation at H3K9, H4K8, and H3K56 play a role in the direct and indirect response to DNA damage induced by an alkylating agent, methyl methanesulphonate (MMS). Artemisinin, the first line antimalarial chemotherapeutics elicits a similar response compared to MMS which suggests its activity as a DNA damaging agent. Moreover, in contrast to the wild-type P. falciparum, two strains (Dd2 and W2) previously shown to exhibit a mutator phenotype, fail to induce their DNA repair upon MMS-induced DNA damage. Genome sequencing of the two mutator strains identified point mutations in 18 DNA repair genes which may contribute to this phenomenon.

Biochemical and functional characterization of Plasmodium falciparum DNA polymerase δ

Background

Emergence of drug-resistant Plasmodium falciparum has created an urgent need for new drug targets. DNA polymerase δ is an essential enzyme required for chromosomal DNA replication and repair, and therefore may be a potential target for anti-malarial drug development. However, little is known of the characteristics and function of this P. falciparum enzyme.

Methods

The coding sequences of DNA polymerase δ catalytic subunit (PfPolδ-cat), DNA polymerase δ small subunit (PfPolδS) and proliferating cell nuclear antigen (PfPCNA) from chloroquine- and pyrimethamine-resistant P. falciparum strain K1 were amplified, cloned into an expression vector and expressed in Escherichia coli. The recombinant proteins were analysed by SDS-PAGE and identified by LC–MS/MS. PfPolδ-cat was biochemically characterized. The roles of PfPolδS and PfPCNA in PfPolδ-cat function were investigated. In addition, inhibitory effects of 11 compounds were tested on PfPolδ-cat activity and on in vitro parasite growth using SYBR Green I assay.

Results

The purified recombinant protein PfPolδ-cat, PfPolδS and PfPCNA showed on SDS-PAGE the expected size of 143, 57 and 34 kDa, respectively. Predicted amino acid sequence of the PfPolδ-cat and PfPolδS had 59.2 and 24.7 % similarity respectively to that of the human counterpart. The PfPolδ-cat possessed both DNA polymerase and 3′–5′ exonuclease activities. It used both Mg²⁺ and Mn²⁺ as cofactors and was inhibited by high KCl salt (>200 mM). PfPolδS stimulated PfPolδ-cat activity threefolds and up to fourfolds when PfPCNA was included in the assay. Only two compounds were potent inhibitors of PfPolδ-cat, namely, butylphenyl-dGTP (BuPdGTP; IC₅₀ of 38 µM) and 7-acetoxypentyl-(3, 4 dichlorobenzyl) guanine (7-acetoxypentyl-DCBG; IC₅₀ of 55 µM). The latter compound showed higher inhibition on parasite growth (IC₅₀ of 4.1 µM).

Conclusions

Recombinant PfPolδ-cat, PfPolδS and PfPCNA were successfully expressed and purified. PfPolS and PfPCNA increased DNA polymerase activity of PfPolδ-cat. The high sensitivity of PfPolδ to BuPdGTP can be used to differentiate parasite enzyme from mammalian and human counterparts. Interestingly, 7-acetoxypentyl-DCBG showed inhibitory effects on both enzyme activity and parasite growth. Thus, 7-acetoxypentyl-DCBG is a potential candidate for future development of a new class of anti-malarial agents targeting parasite replicative DNA polymerase.

Molecular characterization of Plasmodium falciparum putative polynucleotide kinase/phosphatase

Polynucleotide kinase/phosphatase (PNKP) is a bifunctional enzyme that can phosphorylate the 5'-OH termini and dephosphorylate the 3'-phosphate termini of DNA. It is a DNA repair enzyme involved in the processing of strand break termini, which permits subsequent repair proteins to replace missing nucleotides and rejoin broken strands. Little is known about DNA repair in Plasmodium falciparum, including the roles of PNKP in repairing parasite DNA. We identified a P. falciparum gene encoding a protein with 24% homology to human PNKP and thus suggestive of a putative PNKP. In this study, the PNKP gene of P. falciparum strain K1 (PfPNKP) was successfully cloned and expressed in E. coli as a GST-PfPNKP recombinant protein. MALDI-TOF/TOF analysis of the protein confirmed the identity of PfPNKP. Assays for enzymatic activity were carried out with a variety of single- and double-stranded substrates. Although 3'-phosphatase activity was detected, PfPNKP was observed to dephosphorylate single-stranded substrates or double-stranded substrates with a short 3'-single-stranded overhang, but not double-stranded substrates that mimicked single-strand breaks. We hypothesize that unlike human PNKP, PfPNKP may not be involved in single-strand break repair, since alternative terminal processing mechanisms can substitute for PfPNKP, and that PfPNKP DNA repair actions may be confined to overhanging termini of double-strand breaks.

Plasmodium falciparum origin recognition complex subunit 5: functional characterization and role in DNA replication foci formation

The mechanism of DNA replication initiation and progression is poorly understood in the parasites, including human malaria parasite Plasmodium falciparum. Using bioinformatics tools and yeast complementation assay, we identified a putative homologue of Saccharomyces cerevisiaeorigin recognition complex subunit 5 in P. falciparum (PfORC5). PfORC5 forms distinct nuclear foci colocalized with the replication foci marker proliferating cell nuclear antigen (PfPCNA) and co-immunoprecipitates with PCNA during early-to-mid trophozoite stage replicating parasites. Interestingly, these proteins separate from each other at the non-replicating late schizont stage, citing the evidence of the presence of both PCNA and ORC components in replication foci during eukaryotic DNA replication. PfORC1, another ORC subunit, colocalizes with PfPCNA and PfORC5 at the beginning of DNA replication, but gets degraded at the late schizont stage, ensuring the regulation of DNA replication in the parasites. Further, we have identified putative PCNA-interacting protein box in PfORC1 that may explain in part the colocalization of PfORC and PfPCNA. Additionally, use of specific DNA replication inhibitor hydroxyurea affects ORC5/PCNA foci formation and parasitic growth. These results strongly favour replication factory model in the parasites and confer great potential to understand the co-ordination between ORC and PCNA during eukaryotic DNA replication in general.